Glutamate is the major amino acid neurotransmitter in the mammalian central nervous system. Glutamate plays a major role in numerous physiological functions, such as learning and memory but also sensory perception, development of synaptic plasticity, motor control, respiration, and regulation of cardiovascular function. Furthermore, glutamate is at the centre of several different neurological and psychiatric diseases, where there is an imbalance in glutamatergic neurotransmission.
Glutamate mediates synaptic neurotransmission through the activation of ionotropic glutamate receptors channels (iGluRs), and the NMDA, AMPA and kainate receptors which are responsible for fast excitatory transmission (Kew and Kemp Psychopharmacol., (2005), 179:4-29).
In addition, glutamate activates metabotropic glutamate receptors (mGluRs) which have a more modulatory role that contributes to the fine-tuning of synaptic efficacy.
Glutamate activates the mGluRs through binding to the large extracellular amino-terminal domain of the receptor, herein called the orthosteric binding site. This binding induces a conformational change in the receptor which results in the activation of the G-protein and intracellular signaling pathways.
mGluR5 and NMDA receptors are co-expressed in hippocampus, cortex and striatum.
mGluR5 potentiates NMDA receptor function via a PKC- and Src-dependent mechanism. Blockade of mGluR5 or NMDA receptors impairs cognitive function whereas activation of mGluR5 or NMDA receptors normalizes amphetamine disrupted pre-pulse inhibition (PPI). Stimulation of mGluR5 receptors is postulated to normalize the NMDA receptor hypofunction in schizophrenia. An mGluR5 positive allosteric modulator (PAM) may have beneficial effects on cognition, positive and negative symptoms of schizophrenia, and cognitive deficits in various forms of dementia and mild cognitive impairment.
To date, most of the available pharmacological tools targeting mGluRs are orthosteric ligands which cross react with several members of the family as they are structural analogues of glutamate and have limited bioavailability (Schoepp D. D. et al. Neuropharmacology (1999), 38(10), 1431-1476). A new avenue for developing selective compounds acting at mGluRs is to identify molecules that act through allosteric mechanisms, modulating the receptor by binding to a site different from the highly conserved glutamate binding site. Positive allosteric modulators of mGluRs have emerged recently as novel pharmacological entities offering this attractive alternative. This type of molecule has been discovered for several mGluR sub-types (reviewed in Mutel (2002) Expert Opin. Ther. Patents 12:1-8).
WO-2005/082856, WO-2007/023242 and WO-2007/023290 (Merz) disclose tetrahydroquinolinones as modulators of Group I mGluRs. WO 2008/151184 (Vanderbilt University) discloses benzamides as mGluR5 positive allosteric modulators. Fused thiazole compounds are further known from amongst others WO-2008/060597 (Vertex), WO-2008/076562 (Lilly), WO-2008/001076 (UCB), WO-2008/066174 (Lilly) and WO-2006/066174 (Eli Lilly). US 2010/0081690 (Addex Pharma, S.A.) published on Apr. 1, 2010 discloses oxazole derivatives as positive allosteric modulators of mGluR5. WO 2008/012010 (UCB Pharma, S.A.) published on Jan. 31, 2008 discloses fused oxazoles and thiazoles as Histamine H3-receptor ligands with groups at the 2-position of the thiazole ring that are different to the ones disclosed herein. WO 2010/114971 (Sepracor Inc.), published on Oct. 7, 2010 discloses bicyclic compounds and provides data for their activity as mGluR5 NAMs; none of the compounds disclosed therein contain a substituent at the 2-position of the thiazole ring as disclosed herein, and none of the exemplified compounds contain a carbonyl group in the bicyclic core.
It is the object of the present invention to provide novel compounds with an improved balance of properties over the prior compounds, in particular, advantageous properties such as central penetration, improved in vivo potency at lower dose and/or improved pharmacokinetic properties.